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First exomoon glimpsed – 1800 light years from Earth

By Jacob Aron

A NEWLY DETECTED celestial body may be the first moon spotted outside our solar system. Massive, far from its parent planet and with no host star, the candidate “exomoon” is unlike any other known moon.

With so many exoplanets already found, the hunt is on for exomoons. Until now, they had proved elusive. “This is the first serious candidate,” says David Kipping of Harvard University, who was not involved in finding it. The mooted moon and its parent planet drift star-less in the cosmos. This makes them unlikely to host life, but some people expect exomoons in general to be more life-friendly than their planets.

The uncertain status of the weird objects stems from how they were detected. As they passed in front of a distant star, their gravity amplified its light first by 70 times and then an hour later by a smaller amount. David Bennett of the University of Notre Dame in Indiana and his colleagues report that they spotted this “microlensing” effect in 2011, using telescopes around the world. It fits with a large object passing in front of the star, followed by a smaller one.

Deducing what the objects are is harder. If they are only about 1800 light years from our solar system, then they are a planet about four times the mass of Jupiter and a moon about half the mass of Earth. But the readings also fit another scenario&colon; a small, or failed, star orbited by a Neptune-mass planet (arxiv.org/abs/1312.3951).

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Detecting the bodies again to determine the truth may be impossible because spotting objects via microlensing requires them to line up in a particular way.

If the planet-moon scenario is correct, then the duo is weird. Not only would the moon be massive, it would also orbit about 20 million kilometres from its planet. For comparison, Jupiter’s moon Ganymede, the largest in the solar system, is about 1 million kilometres out and just 2 per cent Earth’s mass. Strangest of all, the moon and planet have no host star.

Not only would the exomoon be massive, it would also orbit 20 million kilometres from its planet

This article appeared in print under the headline “First teasing glimpse of an alien moon”